[Milsurplus] GRC-109, RS-1 transmitter

[B. Smith]

Has anyone modified the Transmit Receive (TR) circuit for the  T-784(GRC-109 or 
RT-3 (RS-1) to improve receiver sensitivity when using the internal automatic antenna switch between transmit and receive?

The "TR circuit"  is activated  by the transmitter keying circuit  and  basically the receive antenna  input is grounded when the transmitter is keyed.   The  problem is that there are  extreme loses in the receive side of the TR  circuit  due to the  receiver coupling capacitor C14 which is only 18 pf but is in the output circuit of the transmitter.  C-14 is too small in value to adequately pass RF at lower HF frequencies in this circuit ---- but you can not change the value of C-14 without upsetting the transmitter tank circuit and dumping RF into the front end of the receiver. 

The receiver sensitivity loss is paraphrased in TM-11-5820-474-14, Chapter two, 14e, under "Preliminary Connections, Separate Transmitting and Receiving Antennas."

In Vietnam at base camps the sets were used with separate antennas.

I have been using an external antenna change over switch for the  set to prevent the receive loses but I am preparing to modify the set using either pin diodes (ugh) or a small relay.  :-)

73 breck k4che